We had round, simply because I like the softness of round cakes. Square cakes are a bit too 'funky' for me
(I know that makes no sense, but I just don't like hard edges!)
We had caramel mud, choc mud and white choc mud, one flavour per tier. If you are having the different cake at the top, you can really do what you like and what the cake maker can do though - as it won't need to be solid enough to support anything on top of it. Maybe a spounge? What sort of cake does your FH like? I don't like mud cake, but we had a whole cake of mud as we knew it would appeal to the majority of our guests.